The present invention relates generally to gas turbine engines, and, more specifically, to outlet guide vanes therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel for generating hot combustion gases from which energy is extracted in turbine stages. A high pressure turbine (HPT) immediately follows the combustor and extracts energy for powering the compressor. A low pressure turbine (LPT) follows the HPT and extracts additional energy from the combustion gases for powering an upstream fan in an exemplary aircraft turbofan engine application.
Each turbine stage includes a row of nozzle vanes specifically configured for precisely directing the combustion gases into a cooperating row of turbine rotor blades disposed downstream therefrom. The vanes and blades have specifically configured aerodynamic profiles for maximizing energy extraction from the combustion gases, with the profiles thereof being opposite to each other and alternating from stage to stage.
From the last turbine stage in the LPT, the combustion gases are exhausted through outlet guide vanes (OGVs) typically found in the turbine rear frame immediately downstream of the LPT.
The OGVs typically have specific aerodynamic profiles to remove swirl, or deswirl the exhaust flow prior to discharge from the engine for enhancing the performance thereof. Exhaust swirl is defined as the angle of discharge from the last stage turbine blades relative to the axial centerline axis of the engine. The swirl angle will vary during low to high power operation of the engine.
The range or swing in swirl angle varies from minimum to maximum values depending upon the configuration and operation of the specific engine and may be relatively small or relatively large. For small values of swirl range, the individual OGVs may have suitable aerodynamic profiles with generally convex suction sides and generally concave pressure sides, with a corresponding pitch or angular orientation around the radial axis for deswirling the exhaust flow. Deswirling operation of the OGVs remains effective as long as the exhaust flow remains attached to the surfaces of the vanes.
In applications containing large swirl range, the specific aerodynamic profile and angular orientation of the OGVs may be insufficient to prevent flow separation from the vanes at one or both extremes in the range of swirl angles. Since a vane is typically optimized for a specific design point, off-design point operation of the vane changes the aerodynamic performance thereof eventually leading to flow separation at excess swirl angles of the exhaust.
Flow separation of the exhaust flow from the OGVs is undesirable since it destroys the ability of the vanes to properly deswirl the exhaust flow, and therefore reduces aerodynamic performance and efficiency of the engine.
The ability to deswirl exhaust flow is made more difficult in variable cycle gas turbine engines such as those specifically configured for short takeoff and vertical landing (STOVL) operations. STOVL aircraft are typically used by the military for the extreme military requirements thereof. One type of STOVL aircraft includes an augmented turbofan engine having an afterburner at the aft end thereof, with a variable area exhaust nozzle. The afterburner permits additional fuel to be burned therein for substantially increasing the available thrust and power generated by the engine when required.
Since the afterburner is disposed downstream from the turbine OGVs, performance of those vanes is further important to ensure suitably deswirled exhaust flow to the afterburner for the proper performance thereof during reheat or wet operation.
Performance of the turbine OGVs is further complicated by the modification of the turbofan engine for the STOVL operation which may include an extension of the fan drive shaft for powering an auxiliary fan mounted in the aircraft wing for enhancing vertical lift. And, bleed tubes may join the turbofan bypass duct for bleeding therefrom when desired a portion of the fan air which is diverted to corresponding nozzles in the aircraft for providing additional vertical lift capability and stability control of the aircraft in the STOVL mode of operation.
Accordingly, this exemplary form of STOVL turbofan engine creates a large swing or range in the swirl angle of the exhaust discharged from the core engine through the OGVs. In conventional takeoff and landing operation of the engine, the swirl angle of the exhaust flow is limited in value and range. Whereas, during the STOVL mode of operation of the engine, the swirl angle of the exhaust flow from the core engine is substantially changed to large values.
The typical fixed-design deswirling outlet guide vane is thusly severely limited in its ability to handle the large range of swirl angle change found in a STOVL aircraft engine.
It is therefore desired to provide outlet guide vanes specifically configured for accommodating large swing in swirl without undesirable flow separation therein.